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A severe case of placental mesenchymal dysplasia occurred in association with intrauterine fetal death (IUFD). The gravida-1, para-1 mother was a 26-year-old Japanese. The first pregnancy was unremarkable and a healthy female infant was delivered. The present pregnancy had been uneventful until 34 weeks of gestation when IUFD was detected. The 1516-g (mean ± SD, 2050 ± 387 g) stillborn infant had no external abnormalities and the karyotype was 46,XX. The placenta was markedly enlarged (1050 g; mean ± SD, 452 ± 202 g), and approximately 80% was occupied by extraordinary enlarged villous structures with a myxoid appearance. Histologically, the dysplastic villi had myxoid stroma and a decreased number of, occasionally obliterated, fetal vessels. There was no abnormal trophoblastic proliferation. Large-sized fetal vessels in the chorionic plate frequently contained organized thrombi. This is the first case of placental mesenchymal dysplasia, which possibly lead to the IUFD.